Types of light sources for exposure apparatuses used in the manufacture of semiconductors have evolved from g-line with a wavelength of 436 nm, through i-line with a wavelength of 365 nm and a KrF laser with a wavelength of 248 nm, into an ArF laser with a wavelength of 193 nm, gradually shortening the wavelengths. Also, EUV lithography using extreme ultraviolet (EUV) with a wavelength of about 13.5 nm has been offered in order to achieve the finer pattern transfer. In the EUV lithography, a reflective mask is used partly because a difference in absorptivity of EUV light between materials is small. For example, a reflective mask, in which a multilayer reflective film for reflecting exposure light is formed on a substrate and a phase-shift film for absorbing the exposure light is formed in a pattern on a protective film for protecting the multilayer reflective film, has been proposed. The light incident on the reflective mask mounted on an exposure machine (pattern transfer apparatus) is absorbed at a region where the phase-shift film pattern exists, and is reflected by the multilayer reflective film at a region where the phase-shift film pattern does not exist, such that an optical image is transferred onto a semiconductor substrate through a catoptric system. In the phase-shift film pattern, some of the incident exposure light is reflected with a phase difference of about 180 degrees with respect to the light reflected by the multilayer reflective film (phase shift), thereby achieving the contrast.
The technique related to reflective masks for EUV lithography and mask blanks for manufacturing the reflective masks is disclosed in, for example, Patent Documents 1 to 4.